Method for preparing the base body of a reflecting mirror

ABSTRACT

The invention provides a method of preparing a light-weight base body of a reflecting mirror such as a mirror in a reflecting astronomical telescope, which is very stable not only thermally but also mechanically and is highly resistant against adverse environmental influences. The mirror base is composed of a porous foamed disc body of fused quartz glass or high-silica glass sandwiched by two plates of fused quartz glass or high-silica glass, of which one is made from transparent glass and serves to provide an optical surface having flatness or a specified curvature, and the side surface of the porous foamed disc body is protected against environmental influences by providing an air-tight sealing layer formed from a silicone rubber-based sealing agent or a thin sheet of fused quartz glass or high-silica glass.

BACKGROUND OF THE INVENTION

The present invention relates to a base body of a reflecting mirror anda method for the preparation thereof. More particularly, the inventionrelates to a base body of a reflecting mirror such as those inastronomical telescopes, for beam collimation or diffusion in the spaceindustry and so on, which is characterized by outstandingly light weightto ensure good operability and still has high mechanical strengths as astructural body and optical characteristics, as well as a method for thepreparation thereof.

In the prior art, high-precision reflecting mirrors used in astronomicaltelescopes or for optical collimation of high-energy light beams arefabricated by providing a mirror base made from bubble-free fused quartzglass or high-silica glass having an extremely small thermal expansioncoefficient with a high-reflectivity vapor-deposited film of a metalsuch as aluminum on one of the surfaces with flatness or a specifiedspherical curvature. These reflecting mirrors are used by beingsupported on a supporting stand to ensure free rotation and movement asdesired. It is essential in these reflecting mirrors, especially whenthe size thereof is large, that the dimensional accuracy or precision ofthe reflecting surface is not influenced by the changes in thetemperature or condition of the mechanical forces acting thereon ascaused by the change in the disposition of the mirror.

The above mentioned requirements for high-precision reflecting mirrorscan be satisfied relatively easily when the mirror is small having adiameter of, for example, 20 cm or smaller. In recent years, however,demand for high-precision reflecting mirrors is rapidly expanding forthose having a larger and larger diameter of, for example, 1 meter oreven larger. Such a large-sized reflecting mirror naturally has a verylarge weight which is responsible for the deformation of the mirror bodycaused by the influence of the disposition such as the mounting angle,resulting in warping or undulation of the mirror surface to greatlydecrease the optical performance of the reflecting mirror.

Besides the above mentioned mechanical deformation due to the weight ofthe mirror body per se, large-sized reflecting mirrors also have aproblem of dimensional expansion and shrinkage caused by the change inthe ambient temperature or as a result of the radiation of high-energylight beams so that the mirror surface is subject to warping orundulation, resulting in a decrease in the optical performance of thereflecting mirror. This is the reason for the use of fused quartz glassor high-silica glass having an outstandingly small thermal expansioncoefficient as the material of reflecting mirrors.

These glassy materials, however, have a relatively large specificgravity so that the mirror base shaped from glass is so heavy that theoperability of the reflecting mirror is unavoidably poor if not tomention the increased mechanical deformation due to the large bodyweight of the mirror. Accordingly, various attempts and proposals havebeen made for decreasing the body weight of a reflecting mirror by theimprovement in the supporting structure of the surface plate of themirror without sacrifice in the supporting strength to comply with thepractical requirement to ensure good operability of a large-sizedreflecting mirror having a glass-made mirror base by decreasing theweight of the mirror base.

For example, Japanese Patent Publication 63-57761 discloses alight-weight glass-made mirror base of a reflecting mirror forastronomical telescopes, which consists of a front plate, i.e. thesurface plate for providing the reflecting surface by metal plating, arear plate or backing plate as a base for supporting the front plate anda latticework composed of a plural number of rows of pipes made fromfused quartz glass sandwiched by the two plates. In the latticework ofpipes, each pipe of the pipe rows is contacted in a cross-stitcharrangement with the two pipes in the respective adjacent two rowsforming contacting lines or contacting zones while the wall thickness ofthe pipes is smaller along the above mentioned contacting lines or zonesthan in the other portions of the pipe walls and the pipes are joinedtogether into an integral latticework by welding along the contactinglines or zones. Such a complicated latticework structure of theintermediate layer between the front plate and the rear plate, however,is industrially very disadvantageous because of the very large costs forthe preparation thereof. In addition, the mirror base having such alatticework structure has poor mechanical strength in the directionwithin the surface plane not to withstand the high-precision lapping andpolishing works of the optical surface before metal plating to have aspecified flatness or curvature of the surface.

Moreover, it is a very difficult matter to obtain the pipe elementsforming the latticework having an exactly equal effective height so thatthe front plate supported by the latticework unavoidably retains astrain corresponding to the height difference in the pipe elementsforming the latticework to cause deformation or undulation of thereflecting surface after lapse of some length of time. The rigidity ofsuch a latticework is of course inherently anisotropic and differsbetween the directions which may be perpendicular to or parallel withthe reflecting surface so that the reflecting mirror having such a basebody can hardly be used when the mirror must take various dispositionsby being rotated or moved on the supporting stand due to the pooraccuracy of the reflecting surface when the disposition of the mirror isvaried.

Further, Japanese Patent Publication 61-26041 discloses anotherlight-weight glass-made base body of a reflecting mirror forastronomical telescopes. The base body of fused quartz glass alsoconsists of a front plate, a rear plate and an interposed latticeworktherebetween integrated into a body by welding. The latticework isprepared by putting plate-formed and/or tubular lattice elements on asupporting plate to form a lattice and filling the spaces formed betweenthe lattice elements with tiny pieces of the same glass susceptible tosintering followed by heating to effect sintering this assemblage asfastened with a graphite ring in a furnace under a non-oxidizingatmosphere. The thus prepared latticework is sandwiched between thefront plate and the rear plate and welded together into an integral basebody to be finished by polishing the surface of the front plate. Such abase body of a reflecting mirror is industrially disadvantageous and notpractical due to the very lengthy and troublesome procedure ofmanufacture with very high costs, in addition to the problem that thefront plate bonded to the latticework by welding retains substantialstrains at the welded portions to greatly affect the dimensionalaccuracy of the reflecting surface.

SUMMARY OF THE INVENTION

The present invention accordingly has an object to provide a method forthe preparation of a light-weight base body of a reflecting mirrorhaving excellent dimensional stability against the influences oftemperature changes and gravitational weight of the body per se, toavoid occurrence of deformation of the reflecting surface even when thesize of the mirror is quite large. Another object of the invention is toprovide a method for the preparation of a light-weight base body of areflecting mirror having high three-dimensional mechanical strengths notonly in the direction perpendicular to the reflecting surface but alsoin the direction parallel with the reflecting surface. A further objectof the invention is to provide a method for the preparation of alight-weight base body of a reflecting mirror which is protected on theouter surface with an air-tight sealing layer to avoid contamination inthe processing procedure of polishing of the surface and subsequenthandling.

Thus, the base body of a reflecting mirror provided by the inventivemethod is an integral body which comprises:

(A) a front plate of transparent fused quartz glass or high-silica glasshaving an optically flat or curved surface;

(B) a porous foamed body of fused quartz glass or high-silica glassbonded to the surface of the front plate opposite to the optically flator curved surface;

(C) a rear plate of fused quartz glass or high-silica glass bonded tothe surface of the porous foamed body opposite to the front plate; and

(D) an air-tight sealing layer on the side surface of the porous foamedbody.

The method of the invention for the preparation of a light-weight basebody of a reflecting mirror comprises the steps of:

(a) bonding by fusion a porous foamed disc body of fused quartz glass orhigh-silica glass having a bulk density in the range from 0.1 to 1 g/cm³on one surface with a front plate of transparent fused quartz glass orhigh-silica glass having an optically flat or curved surface at thesurface opposite to the optically flat or curved surface;

(b) bonding by fusion a plate of fused quartz glass or high-silica glassto the surface of the porous foamed disc body opposite to the surface towhich the front plate is bonded; and

(c) bonding an air-tight sealing layer to the side surface of the porousfoamed disc body.

It is preferable that the air-tight sealing layer bonded to the sidesurface of the porous foamed disc body is formed from a siliconerubber-based sealing agent or, more preferably, from fused quartz glassor high-silica glass which is bonded by fusion to the side surface ofthe disc.

BRIEF DESCRIPTION OF THE DRAWING

The figure is a perspective view of the inventive base body of areflecting mirror as partially cut open.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In step (a) of the above defined inventive method, a front plate orsurface plate, which has an optically flat or curved surface to beplated with a high-reflective metal to form a reflecting surface, isbonded by fusion to a porous foamed body as the base. This front plateis made from fused quartz glass or high-silica glass substantially freefrom bubbles to ensure very small thermal expansion or shrinkage not tobe dimensionally influenced by the change in temperature. This frontplate is shaped to have flatness or specified curvature depending on theintended use of the reflecting mirror.

The above described front plate is supported by a porous foamed discbody as the base. The material of the porous foamed disc body is fusedquartz glass or high-silica glass of which the content of silicondioxide is preferably at least 99% by weight. The porous foamed discbody should have a bulk density in the range from 0.1 to 1 g/cm³. Whenthe bulk density thereof is too low, the disc body would have poormechanical strengths not to be suitable as a supporting base of thefront plate with stability. When the bulk density thereof is too high,on the other hand, the body weight of the mirror base prepared by theinventive method cannot be small enough as a matter of course so thatthe reflecting mirror would be more or less subject to the problem ofmechanical deformation by the gravity of the mirror body per se, inaddition to the decreased operability of the reflecting mirror.

The porosity of the porous foamed body should desirably consist mainlyof closed cells. When the closed cells form a well-developedthree-dimensional network structure, the porous foamed body would beimparted with a high compressive strength in every direction againstcompressive forces.

The porous foamed disc body is bonded to the front plate over the wholearea of the surface opposite to the optically flat or curved surface toserve as a uniform support so that high resistance is exhibited againstthe pressing force added to the surface of the front plate in thepolishing work of the optical surface. In addition, the porous foameddisc body is highly resistant against mechanical forces in the directionparallel to the surface of the front plate.

The porous foamed disc body of fused quartz glass or high-silica glasscan be prepared according to a procedure known in the art. For example,a powder of fused quartz glass consisting of silicon dioxide havinghydroxyl groups is heated in an atmosphere of ammonia to be reactedtherewith followed by shaping into a desired form and sintering.Alternatively, a powder of fused quartz glass is first shaped into aform and sintered and the sintered body is then ammoniated by heating inan atmosphere of ammonia. Thereafter, the ammoniated sintered body isheated in an electric furnace at a temperature of 1500° to 1800° C. tocause softening or melting of silicon dioxide which is expanded by thegas evolved from the glass to give a porous foamed body mainlyconsisting of closed cells. Further alternatively, foamed porous bodiesof glass can be prepared by heating a blend of a glass powder and ablowing agent at a temperature sufficiently high to cause decompositionof the blowing agent to evolve a gas and to cause softening of the glasspowder. At any rate, it is important in these processes that theconditions of foaming should be selected so as to obtain closed cellshaving an adequate diameter and to prevent formation of open cells byexcessively increasing the temperature.

The porous foamed body of glass obtained in the above described manneris cut into a desired form such as a disc or square or rectangular boarddepending on the size and form of the reflecting mirror to be preparedtherefrom. The thus shaped body of porous foamed glass is bonded, on onesurface, to a front plate of transparent fused quartz glass orhigh-silica glass to provide the optical surface and, on the othersurface, to a rear plate of also fused quartz glass or high-silica glassof which, however, the quality or purity need not be so high as in thefront plate. The front plate and the rear plate can be bonded by fusionto the porous foamed disc body either separately or simultaneously. Theprocedure for bonding of the porous foamed disc body to the front plateor rear plate is as follows. Thus, the porous foamed disc body and theglass plate are laid one on the other by sandwiching an interposed layerof a finely divided silica powder in a thickness of, for example, about1 mm over the whole area and they are heated together for about 1 toabout 4 hours at a temperature higher than the softening point of thesilica powder which, of course, should be higher than the softeningpoint of the porous foamed body and the glass plate. When the softeningpoint of the silica powder is the same as or higher than that of theporous foamed body or glass plate to be bonded together, softening ofthese members takes place to cause deformation or collapsing of theclosed cells before the silica powder is softened to exhibit theadhesive bonding effect. Examples of suitable silica powders includethose prepared by the so-called solgel method or by the pyrolyticgas-phase method in respect of the relatively low melting point. Thesilica powder should have a particle size as fine as possible in respectof the increased uniformity in the fusion-bonding layer formedtherefrom. Practically, it is preferable to use a silica powder havingan average particle diameter not exceeding 10 μm.

By this bonding method, the porous foamed disc body and the front plateor rear plate can be adhesively bonded over the whole contacting areawith very high stability of bonding. In this regard, it is importantthat the surface of the porous foamed disc body is shaped to have aflatness or curvature just to fit the surface of the front plate to bebonded thereto opposite to the optical surface so that contacting can beobtained therebetween substantially over the whole area of surfaces.

The front plate bonded to one surface of the porous foamed disc bodyshould be made from transparent fused quartz glass or high-silica glassof as high as possible purity substantially free from bubbles. This isbecause even extremely fine bubbles may be responsible for distortion ordimensional inaccuracy of the optical surface not to give ahigh-precision reflecting mirror. On the contrary, the rear plate bondedto the other surface of the porous foamed disc body need not be of sohigh purity and transparency as in the front plate and a small number ofbubbles or a somewhat decreased transparency is permissible providedthat it is made from fused quartz glass or high-silica glass.

The porous foamed disc body to which the front plate and the rear plateare bonded at one and the other surfaces is then provided with anair-tight sealing layer on the side surface thereof. Formation of thisair-tight sealing layer is preferably preceded by a smootheningfinishing treatment all over the side surface of, in particular, theporous foamed disc body in order to have the air-tight sealing layerproperly formed. This smoothening treatment of the surface can beperformed either by grinding with an abrasive grinder or by blowing aflame such as an oxyhydrogen flame to the surface.

The thus smoothened side surface of the porous foamed disc body is thenprovided with an air-tight sealing layer. The sealing layer can beformed by coating the smoothened surface with various kinds of sealingagents. An example of suitable sealing agent is a silicone rubber-basedsealing agent which is applied to the surface and cured at roomtemperature or by heating. It is not always necessary that theair-tightness of the sealing layer is very high. However, it has beendiscovered unexpectedly that the air-tight sealing layer on the sidesurface plays a very important role in order to obtain quitesatisfactory results in the precision polishing of the optical surfaceof the front plate of the base body prepared by the inventive method.The reason therefor is not clear but it is presumable that, in thepolishing work using an aqueous dispersion of an abrasive powder, waterand the abrasive particles may enter the pore structure of the porousfoamed disc body and the interstices between the porous disc body andthe front and rear plates, though in very small amounts, to delicatelyinfluence the accuracy of the polishing work on the optical surface towhich a highly reflective layer of a metal such as aluminum and silveris deposited to give a reflecting surface. The base body of a reflectingmirror prepared in the above described manner is then subjected tohigh-precision polishing of the optical surface to be imparted withflatness or a specified curvature depending on the intended use of thereflecting mirror before the optical surface is provided with areflecting layer of a metal. The polishing work of the surface can beperformed according to a conventional procedure, for example, by usingan aqueous dispersion of an abrasive fine powder.

The base body of a reflecting mirror prepared by the inventive method isillustrated in the figure of the accompanying drawing which is aperspective view thereof as partially cut open. The base body isconstituted of a porous foamed disc body 1 of fused quartz glass orhigh-silica glass sandwiched between a front plate 2 having an opticallyfinished surface 2' and a rear plate 3 each also of fused quartz glassor high-silica glass while the side surface of the porous foamed discbody 1 is covered with an air-tight sealing layer 4.

Instead of forming the air-tight sealing layer by using a siliconerubber-based sealing agent, still better results could be obtained whenthe sealing layer was formed from fused quartz glass or high-silicaglass to thus envelop the porous foamed disc body entirely with a glassylayer. Such a sealing layer of glass can be formed by first depositing alayer of a fine glass powder onto the surface to be protected followedby fusion of the powder layer, but it is more practical that a thinsheet of glass is applied to the side surface of the porous foamed discbody and then softened by heating and bent successively from thestarting end to be fusion-bonded to the side surface of the porous discbody. As compared with the sealing layer of a silicone rubber-basedsealing agent, the sealing layer of glass thus formed is veryadvantageous because not only the side surface of the porous body can beprotected almost perfectly against contamination in the polishing workby using an aqueous dispersion of an abrasive powder, but also the basebody of a reflecting mirror can be greatly reinforced in respect of themechanical strengths.

A problem in the base body having the above mentioned glassy sealinglayer on the side surface is that, when the base body after finishing ofthe optical surface by polishing is subjected to the process ofdeposition of the reflecting layer of a metal at an elevatedtemperature, the base body is sometimes heated to a considerably hightemperature so that expansion of the air confined in the base body mayhave an adverse influence on the dimensional accuracy of the bodyeventually leading to bursting of the sealing layer. This problem can bereadily solved by forming one or several small vent holes in theair-tight sealing layer of glass on the side surface.

In the following, the base body and the method of the present inventionare described in more detail by way of examples.

EXAMPLE 1

Silicon tetrachloride was introduced into a burner of oxyhydrogen flameand subjected to flame hydrolysis to form silica soot which was reactedwith ammonia gas at 1000° C. for 2 hours. The thus ammoniated silicasoot was shaped into a form and heated at 1600° C. for 10 minutes sothat the shaped body was expanded by the ammonia gas isolated from theammoniated silica soot to give a porous foamed body of fused quartzglass having a bulk density of about 0.3 g/cm³. This porous foamed bodywas fabricated by cutting into a disc having a diameter of 350 mm and athickness of 25 mm.

A finely divided silica powder was spread on the surface of the thusprepared porous foamed disc body to form a thin powder layer of uniformthickness. Then a transparent fused quartz glass sheet free from anybubbles having a diameter of 350 mm and a thickness of 0.5 mm as a frontplate was laid on the layer of the finely divided silica powder andheated at a temperature of about 1300° C. for 30 minutes so that thesilica powder was melted and the glass sheet was fusion-bonded to theporous foamed disc body. In this heating treatment, the porous foameddisc body was mounted on a plate of fused quartz glass having a diameterof 350 mm and a thickness of 0.5 mm as a rear plate with an interposedlayer of a finely divided silica powder therebetween over the wholesurface so that this rear plate was also bonded simultaneously to theporous foamed disc body.

The side surface of the porous foamed disc body sandwiched between thefront and rear plates was ground smoothly using a grinder and thenuniformly coated with a room temperature-curable, silicone rubber-basedsealing agent which was cured by standing at room temperature to form anair-tight sealing layer having a thickness of 0.5 to 5 mm thereon.

The optical surface of the front plate of the thus prepared base body ofa reflecting mirror could be lapped and polished using an aqueousdispersion of an abrasive powder without the disadvantage of occurrenceof strain to cause a decrease in the performance of light reflectionbecause the front plate was very firmly and uniformly bonded to thesupporting base of the porous foamed disc body over the whole surfaceand was free from deformation or breakage of the front plate by thepressing force added thereto in the polishing work. The flatness of thethus polished optical surface was examined by the method of opticalinterference fringes to find that the interference fringes were allparallel to each other, indicating that the optical surface hadextremely high flatness.

EXAMPLE 2

The procedure for the preparation of a porous foamed disc bodysandwiched by a front plate and a rear plate was substantially the sameas in Example 1 except that the front plate of fused quartz glass had adiameter of 350 mm and a thickness of 3 mm and the rear plate of fusedquartz glass also had a diameter of 350 mm and a thickness of 3 mm. Asheet of fused quartz glass having a thickness of 1 mm, width of 30 mmand length of 1100 mm was applied to the side surface of the porousfoamed disc body and bent and fusion-bonded to the side surfacesuccessively from the starting end by using an oxyhydrogen flame to forman integrated airtight sealing layer.

Flatness of the optical surface was as good as in Example 1 as examinedby the method of optical interference fringes to give parallelinterference fringes after polishing of the surface of the front plateusing an aqueous dispersion of an abrasive powder.

A high-precision flat reflecting mirror was obtained by depositing athin aluminum layer on the polished surface of the front plate. Thisreflecting mirror was much lighter in weight than conventionalreflecting mirrors of the same size, of which the mirror base is formedfrom a block of fused quartz glass, to ensure good operability withoutsacrifice in the optical characteristics.

What is claimed is:
 1. A method for the preparation of a light-weightbase body of a reflecting mirror which comprises the steps of:(a)bonding by fusion a pre-shaped porous foamed disc body of fused quartzglass or high-silica glass having a bulk density in the range from 0.1to 1 g/cm³ on one surface with a front plate of transparent fused quartzglass or high-silica glass having an optically flat or curved surface atthe surface opposite to the optically flat or curved surface; (b)bonding by fusion a plate or fused quartz glass or high-silica glass tothe surface of the porous foamed disc body opposite to the surface towhich the front plate is bonded; steps (a) and (b) being carried outseparately or simultaneously; and (c) after steps (a) and (b) arecompleted, then bonding on air-tight sealing layer to the side surfaceof the porous foamed disc body, which air-tight sealing layer is formedby applying a sheet of fused quartz glass or high-silica glass to theside surface and heating the sheet of fused quartz glass or high-silicaglass to cause bending and fusion-bonding of the sheet to the sidesurface of the porous foamed disc body.